1. Field of Invention
This invention relates to selective hydrogenation catalysts, more particularly to improved, Group IB-promoted palladium catalysts with high pore volume and unique pore volume distribution. The catalysts are designed for the selective hydrogenation of butadiene, acetylenes, diolefins, and trace quantities of other such highly unsaturated hydrocarbon impurities in an olefinic feed stream, particularly in a raw gas feed stream containing C2, C3, C4, C5 and trace quantities of C6 and higher hydrocarbons. This invention also relates to processes of preparation of these catalysts.
2. Prior Art
The manufacture of unsaturated hydrocarbons usually involves cracking various types of hydrocarbons. This process often produces a crude product containing hydrocarbon impurities that are more unsaturated than the desired product. This is particularly a problem with raw gas feed streams from cracking facilities containing C2, C3, C4, C5 and trace quantities of C6 and higher hydrocarbons as well as hydrogen and methane. These raw gas feeds can contain significant unsaturated hydrocarbon impurities, such as 1,3 butadiene, methyl acetylene, propadiene, acetylene, isoprene, and trace quantities of other such unsaturated hydrocarbon impurities.
These unsaturated hydrocarbon impurities are often very difficult to remove completely by fractionation from a hydrocarbon feed stream. Further, it is often difficult, industrially, to hydrogenate these highly unsaturated hydrocarbon impurities without significant hydrogenation of the desired unsaturated hydrocarbons also occurring.
Two general types of gas phase selective hydrogenation processes for removing undesired, highly unsaturated hydrocarbons are commonly used: “front-end” hydrogenation and “tail-end” hydrogenation. “Front-end” hydrogenation involves passing the crude gas from the initial cracking step, often after removal of steam and condensable organic material, over a hydrogenation catalyst. The crude gas generally includes a relatively large volume of hydrogen and a mixture of unsaturated hydrocarbons. Among these products in raw gas feed streams may be C2, C3, C4, C5 and trace quantities of C6 and higher hydrocarbons and may be wet or dry. Typically, the hydrogen gas concentration is greater than the stoichiometric amount needed for complete hydrogenation of the impurities that are present in the crude gas. To minimize the risk of the excess hydrogen gas hydrogenating ethylene in the feed stream, the hydrogenation catalyst must be very selective. In addition, the catalyst risks being damaged in front-end reactions because hydrogenation of ethylene can lead to thermal excursion, known as “run-away”, whereby high temperatures are experienced. Run-away can also result in severe loss of ethylene.
In tail-end hydrogenation, the crude gas is fractionated prior to hydrogenation resulting in concentrated product streams. Hydrogen is then added to these product streams, if necessary, such that a slight excess of hydrogen is present over the quantity required for complete hydrogenation of the impurities. In tail-end systems there is a greater tendency for deactivation of the catalyst, and consequently, periodic regeneration of the catalyst is necessary. While the quantity of hydrogen added can be adjusted to maintain selectivity, formation of polymers is a major problem in this process.
One catalyst that is preferred for selective hydrogenation reactions contains palladium supported on a low surface area carrier, such as a low surface area alumina. However, one of the problems with supported palladium catalysts is that under normal operating conditions not only are the impurities hydrogenated, but a substantial portion of the ethylene is also converted to ethane. In addition, these palladium on alumina catalysts often have relatively low stability over extended periods of time due to the formation of large quantities of oligomers on the catalyst surface. The rate of oligimerization is especially high when butadiene is present in the feedstream mixture. Further these palladium catalysts may not perform at acceptable levels when methyl acetylene, butadiene, isoprene, and other highly unsaturated compounds are present. For these reasons, these heavier compounds are normally removed by distillation prior to contact of the feed mixture with the catalyst.
Enhancers are often added to the palladium to improve the catalysts properties. Copper, silver, gold, germanium, tin, lead, rhenium, gallium, indium, and thallium have been proposed as enhancers or modifiers for such palladium hydrogenation catalysts.
Acetylene hydrogenation catalysts for ethylene purification comprising palladium with a silver additive on a low surface area support material are disclosed in U.S. Pat. Nos. 4,404,124, 4,409,410, 4,484,015, 5,488,024, 5,489,565, 5,648,576, 6,054,409 and CN 1299858. Specifically, U.S. Pat. No. 6,054,409 discloses a catalyst for selective gas phase hydrogenation of acetylenic compounds containing two or three carbon atoms to the corresponding ethylenic compounds. The catalyst comprises palladium, at least one metal from group IB, optionally at least one alkali or alkaline-earth metal and alumina, in which at least 80% of the palladium and at least 80% of the element from group IB are present at the periphery of the catalyst, and wherein the IB metal/palladium ratio is 0.4 to 3 by weight.
In addition, U.S. Patent application 2002/0165092 discloses a catalyst for selective hydrogenation comprising palladium and a group IB metal promoter on an inorganic oxide support, wherein the active components are uniformly distributed between the surface and a depth of more than 300 microns. The catalyst is particularly applicable for feed streams containing C2-C3 fractions, hydrogen and CO.
In addition, U.S. Pat. No. 5,648,576 discloses a selective hydrogenation catalyst for acetylene compounds comprising from about 0.01 to 0.5 weight percent of palladium and from about 0.001 to 0.02 percent by weight of silver. Eighty percent (80%) or more of the silver is placed within a thin layer near the surface of the carrier body.
Catalysts containing palladium and Group IB metals (Cu, Ag, Au) on alumina used for the hydrogenation of acetylenes and diolefins have also been suggested by G.B. 802,100 and U.S. Pat. No. 2,802,889.
Selective hydrogenation catalysts of the prior art comprising palladium with a silver additive often do not exhibit the necessary selectivity and frequently cause significant loss of valuable olefins from the feed stream. This loss is especially a problem with prior art selective hydrogenation catalysts used in raw gas feed streams comprising hydrogen, methane, carbon monoxide and C4, C5, C6 and higher hydrocarbons, which may be wet or dry.
Accordingly, it is an object of this invention to disclose a catalyst useful for selective hydrogenation of a C2, C3, C4, C5, C6 and higher olefinic feed stream containing various acetylenic and diolefinic impurities.
It is a still further object of the invention to disclose a selective hydrogenation catalyst containing palladium supported on an inorganic support with a Group IB additive having a high pore volume and a unique pore volume distribution.
These and other objects can be obtained by the selective hydrogenation catalyst and the process for the preparation of the selective hydrogenation catalyst for use in olefinic feed stream containing acetylenic and diolefinic impurities, particularly raw gas feed streams and particularly for front end selective hydrogenation reactions, which are disclosed by the present invention.